Tank Balancing is Challenged

Blizzard has been making some sweeping changes related to tanking, recently - all in the spirit of making them closer balanced to each other. The recent spate of changes managed to address earlier offsets that were creating imbalances. However, at a fundamental level, Blizzard is going to remain challenged when it comes to balancing tanking classes, truly.
At it's most basic level, you see course corrections such as the recent changes to the druid tanks whom were having a level of survivability that was unauthorized! Initially, Blizzard ratcheted up boss damage in Beta in order to do damage to Feral tanks; however, it was killing the other tanks. They have nerfed the Feral tank, and can therefore reduce the intensity and frequency of boss damage. In fact, they homogenized (overused word lately) the tanks such that now all tanks have 12 second CDs and a damage mitigation "oh crap!" button that brings 50% mitigation to the fight. All tanks, same way across the board.
The real problem arises when making sure that any of the tanking classes can tank equally, that classes bring "extras" which make them unequal from the start. So we've figured out how to make sure that all of the tanks generate about the same threat per second, can all invoke 50% damage mitigation, can all pop 12 second CDs. Warriors still, however, do not have heals or battle-rezes. And while DK tanks do have a degree of self heals, they are no paladin nor druid when it comes to cross-role raid functionality.
Now add to the mix that Shamans now get +30% threat and -5% damage when using Rockbiter, and combine that with a legacy of being able to tank instances, and the general trend of bringing back Vanilla and BC flavors of mechanics (e.g. crowd control) and you have to at least raise an eyebrow at the chance that Shamans might be able to pinch-tank or even outright tank a 5-man (remember that most classes will have approx similar health pools).
The "preventer" to Shamans becoming tanks [again] is damage mitigation. GC recently blue-posted (when making comments about why the recent changes to tanking mechanics and class changes) that damage mitigation will be the most critical aspect of tanking in Cataclysm. For a company that wants to break down the stereo-types of warriors as the best MT, they are driving home that in Cataclysm, your best class for MT role is a warrior - THE best mitigation tank in the game. Then again, they do not have heals, or BRezzes, or other whiz bang spells.
However, we are back at the problem of giving one class an edge because it has a lot of tanking tools in the tool box but no tools from any of the other raid roles, versus making all of the tanks viable for any encounter so that you "bring the player, not the class."
All in all, "balance" is a shifty term interpreted by many in many ways. There is balance across a role in a raid such that all classes with specs designed to fulfill that role can fulfill it fully. There is balance across classes such that one class is not more powerful in a specific role or by virtue of "mixing roles" on the fly (eg. tanks healing). There is balance in the populations of each class where too few of a specific class highlights that something is amiss with its playability.

ps. When Cata lands, Taurens will be the king of tanks - name another race that can be a druid, DK, warrior and paladin? Interestingly, it also makes them the only class that can be all of the healing classes: priest, shaman, druid and paladin.